Method of and means for receiving signals



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AT TO RNEY INVENTOR JAMES w. CONKLIN BY 7r% W36. J. W. CONKLIN METHOD OFAND MEANS FOR RECEIVING SIGNALS Filed Jan. 25, 1954 March 3L PatentedMar. 31, 1936 UNITED STATES PATENT OFFICE METHOD OF AND MEANS FORRECEIVING SIGNALS tion of Delaware Application January 26, 1934, SerialNo. 708,373

4 Claims.

This invention relates to a method of and means for supplying localcarrier to a received signal modulated Wave to reduce the effects of Uselective fading on heavily modulated signals 5 and the effects ofwaxing and waning in carrier strength on the signal energy resultingfrom demodulation of the wave.

More in particular this invention relates to a simplified method of andmeans for locally supplying a synchronized wave to supplement or enhancethe carrier wave in a modulated telephony signal. The artificial orsupplied carrier is of a steady value and preferably is stronger thanthe modulated carrier and therefore reduces the distorting effect ofselective fading and in general improves the quality of the signalresulting from demodulation by reducing the apparent percentagemodulation.

Systems have been known in the prior art which filter out the carrierfrom the incoming signal modulated wave, amplify the said carrierseparately and recombine it with the original energy to replace orsupplement the normal carrier. In general such systems work very wellbut often necessitate careful adjustments of tuning controls and requirevery selective filtering devices. Such filtering devices are necessarilycostly and from a practical standpoint may not be applicable to standardbroadcast receivers as known today. In these known systems crystalfilters are usually used, said filters being the most selective typeavailable for this purpose. The filters, of course, have the drawbackthat their fixed characteristics limit their use to one particular waveor to a particular intermediate frequency where a superheterodyne typereceiver is used. With present day broadcast practice of using highpercentages of modulation on the carrier, even small amounts ofselective fading increases the appar- 40 ent percentage modulation of.the wave above one hundred percent, and consequently introduces into thesignal bad distortions. The high percentage modulation will, of itself,produce noticeable second harmonic distortions in the general run ofdetector circuits in use even without selective fading. For the abovereasons where high quality of reproduction is desired, some deviceshould in accordance with my invention be incorporated in the receiverto supplement the received carrier wave.

In accordance with my invention, I provide this supplemental carrier bymeans of a vacuum tube oscillator to which a small amount of thereceived signal modulated wave is fed prior to detection thereof for thepurpose of holding the local oscillator in step. The local oscillator istuned as closely as possible to the carrier frequency, in which state avery little energy is required to entrain it or to lock it in step as tofrequency and phase with the incoming carrier. 5' In practical broadcastreceiver the tuning for such an oscillator can be very easily gangedwith the other tuning controls. The output from the oscillator is thenimpressed on the demodulator circuit to supplement therein the incoming10 carrier. The operation of such a device requires no technicalunderstanding of its principles or extraordinary skill, as it would betuned for zero beat, an audible squeal being produced between theincoming signal modulated carrier and the local oscillations when thereceiving circuits and the local oscillator circuits are not inresonance. Many operators of broadcast receivers now unwittingly tunereceivers so that part of the signal spectrum falls outside the band ofthe receiver and they therefore obtain poor quality signals. The squealmentioned above, which appears in tuning the present invention,facilitates such tuning and insures perfect tuning of the receiver whichresults in good quality signals.

A receiver including the novel features of my invention as outlinedabove will also increase the audio output of the receiver, at the sametime decreasing the distortion. While the effective percentage ofmodulation is reduced the audio output of the demodulator is dependenton the product of the carrier wave and side band amplitudes and theeffect of the local oscillator is to increase the amplitude of theapparent carrier.

The novel features of my invention have been pointed out withparticularity in the claims appended hereto.

The nature of my invention and the manner in which the method thereof iscarried out will be better understood from the following detaileddescription thereof and therefrom when read in connection with thedrawing and in which,

Figure 1 shows for purposes of illustration only the essential elementsof a receiver including a radio frequency amplifier and a demodulatorarranged in accordance with my present invention; while,

Figure 2 shows a receiver of the heterodyne type which includes thenovel features of my invention.

Referring to Figure l of the drawing, A indicates an aerial system whichmay pick up a signal modulated wave from a natural medium. The aerialsystem A may be replaced by an incoming line on which the modulated Wavehas been impressed. The signal wave from A is impressed on the inputcircuit of a conventional radio frequency amplifier B. The signalmodulated wave is amplified in B and impressed on a second radiofrequency amplifier C and from the output of the radio frequencyamplifier C is impressed on the input electrodes of a demodulator D. Theamplified carrier wave is also impressed on the inductance I connectedbetween the control grid and anode of the local oscillator O. Theinductance I of the oscillator O is tuned by condenser K to a normalfrequency substantially equal to the mean frequency of the receivedwave. The oscillator 0 produces oscillations which due to thecontrolling effect of the energy from C acting through the inductance Ion O are in synchronism as to frequency and phase with the signal wave.These oscillations produced by O are then impressed in the desiredamount on a circuit of the detector D. Preferably they are impressed onthe control grid, cathode circuit of the detector D as shown. The radiofrequency amplifiers and/or detector, and/or the oscillator may be tunedby a common control as shown. The detector output may supply thedemodulated signal energy to any utilization circuit.

The energy impressed on the input of the detector should be of such avalue as to not overload the detector in action and may be either fixedor may be adjustable in common with the manual volume control of theradio frequency amplifier. The second method may give a somewhat greaterrange in output volume.

In operation the oscillator circuit connected with 0 will act as a verysharp filter for the carrier frequency wave and at the same time as anamplifier and an amplitude limiter of the wave produced. The energy forentraining the oscillator 0 may be diverted from the wave received atany point but is preferably diverted from a point ahead of the detectorsince at this point the incoming carrier power will be at its greatestvalue and will be sufficient to insure locking of the oscillator instep.

It may be possible to accomplish in a manner similar results by areceiver of the autodyne type, thus eliminating the use of an extraoscillator. However, when the detector is also used as an oscillator asin the autodyne type of receiver the detector tube is automaticallyloaded up so that poor quality audio signal results. Moreover, due tothis overloading of the tube the detecting action thereof is renderedless sensitive. Furthermore, the regeneration in the oscillationproducing circuits introduces considerable frequency distortion in thedetector by sharpening the tuning of the detector circuit and therebyover-emphasizing the lower frequencies.

My method of and means for eliminating distortion resulting fromover-modulation of a carrier wave followed by selective fading or due tothe effects of waxing and waning in carrier strength is applicable toreceivers of the heterodyne type. For example, a receiver circuit asshown in Figure 2 may be used. In Figure 2, A is the aerial system. B isa radio frequency amplifier. D is a first detector. 0 is an oscillationgenerator which supplies oscillations to beat with the signal modulatedwave to produce intermediate frequency. IFA is an intermediate frequency amplifier and D is the second detector. The radio frequencyamplifier, the first detector, the oscillator and the intermediatefrequency amplifier may be conventional. In Figure 2, however, theoscillator O is normally tuned to a frequency equal to the differencefrequency obtained by beating oscillations from O with the incomingsignal. The intermediate frequency energy from the output of IFA is fedto the inductance I in the oscillations producing circuits of O andentrains O to produce oscillations which are synchronized as to phaseand frequency with the oscillations of intermediate frequency in theoutput of IFA. Energy from the oscillator O of the de sired amplitude isimpressed on the input of the second detector D. The operation of thedemodulator of Figure 2 will be readily understood from the statement ofoperation of the receiver of Figure 1 and need not be repeated here. Thesignals from the output of the second detector tube may be utilized inany manner.

In both arrangements I find no difliculty in holding the oscillators Oin step or in synchronism with the incoming wave or the intermediatefrequency wave. In fact it is nearly impossible to keep the oscillatorsfrom pulling into step with a signal which is spaced from the frequencyto which the oscillators are tuned by a few hundred cycles providingthere is any appreciable coupling between the signal and the oscillator.

In some cases the coupling of the oscillator circuit too closely to thedetector circuit may have the effect of narrowing too much the frequencyband. In this event the coupling may be loosened in any manner. Forexample, the oscillator may be coupled to the detector by way of acoupling tube rather than inductively as shown or any satisfactorycoupling arrangement may be utilized.

Having thus described my invention and the operation thereof, what Iclaim is:

l. The method of reducing the effect of attenuation of a signalmodulated wave during the transmission thereof on the signal componentwhich results from demodulation of said wave which includes the steps ofproducing local oscillations in synchronism as to phase and frequencywith a received Wave, combining said oscillations with the receivedwave, and demodulating the combined energy.

2. Means for demodulating a signal modulated wave including means foreliminating the effect of over-modulation of the wave or of waxing orwaning of the strength of the wave on the signal component resultingfrom said demodulation, comprising signal modulated wave receivingmeans, a local oscillator normally tuned to the frequency of the wavereceived, a coupling circuit interposed between the local oscillator andthe output of the wave receiving means to feed received waves to thelocal oscillator to entrain the same, and a demodulator coupled at itsinput to the local oscillator and to the output of the wave receivingmeans.

3. Means for demodulating signal modulated waves including means foreliminating the effect of over-modulation of the wave or of waxing orwaning of the strength of the wave on the signal component resultingfrom said demodulation, comprising signal modulated wave receivingmeans, a local oscillator comprising a thermionic tube having itselectrodes connected in circuits normally tuned to the frequency of thewave received, and energized to tend to produce oscillations at saidfrequency, means for impressing oscillations from the output of the wavereceiving means on the circuits of said oscillator to excite and entrainthe same, and a demodulator coupled to the local oscillator and to theoutput of the wave receiving means.

4. Means for demodulating signal modulated waves including means foreliminating the effect of over-modulation of said waves or the effect ofwaxing and waning in the amplitude of said waves on the signal componentresulting from said demodulation comprising a signal modulated carrierwave responsive means, a source of local oscillations, a demodulatorcoupled to said signal modulated carrier wave responsive means and tosaid source of local oscillation, an intermediate frequency amplifiercoupled to said demodulating means, a local oscillator comprising athermionic tube having its electrodes interconnected by circuits toproduce oscillations, means for tuning said circuits to a frequencysubstantially equal to said intermediate frequency, a seconddemodulator, said second demodulator having its input electrodes coupledto said last named oscillator and to the output of said intermediatefrequency amplifier, and means for impressing energy from saidintermediate frequency amplifier on said last named oscillator toentrain the same to produce oscillations which are synchronized infrequency and in phase with the oscillations in the output of saidintermediate frequency amplifier.

JAMES W. CONKLIN.

